TON 618 Size Comparison

Size comparison videos usually stack objects side by side and let you guess the distances. This one puts every object where the Sun is and draws the planets’ orbits around it, so you can read its size directly against the Solar System. Nothing is exaggerated: every object is drawn to scale, and each step zooms out to the next object. Press play for the guided tour, or jump to any object with the buttons.

Every object at a glance

For black holes, “radius” means the event horizon: the boundary beyond which nothing, not even light, can escape. For stars, it is the visible surface. One AU is the average distance between Earth and the Sun, about 150 million km.

How big is TON 618 compared to the Solar System?

TON 618 is a quasar about 10 billion light-years away, powered by one of the most massive black holes known, roughly 40.7 billion times the mass of the Sun. Its event horizon has a radius of about 803 AU. Put it where the Sun is and the entire planetary system, out to Neptune at 30 AU, would fit inside it about 27 times over in width. Voyager 1, the most distant spacecraft, is about 170 AU from the Sun in 2026; it would still be deep inside the horizon.

Two numbers make the scale tangible:

• Light takes about 9 days to cross TON 618’s event horizon from one side to the other. It crosses the whole Solar System out to Neptune in about 8 hours.
• Its average density is about 100 times lower than air at sea level (about 0.01 kg/m³ if you spread its mass evenly through the horizon). Bigger black holes are less dense, because their radius grows in step with their mass while their volume grows with its cube.

You will often see TON 618 quoted at 66 billion solar masses and 1,300 AU. That is an older estimate based on a different emission line in the quasar’s spectrum; a 2019 reanalysis of the same data gave the lower 40.7 billion figure used here. Either way, it dwarfs everything else on this page.

Black hole size compared to the Sun and Earth

A black hole’s size depends only on its mass. For a non-rotating black hole the event horizon sits at the Schwarzschild radius:

The radius grows in exact proportion to the mass, so the same formula covers everything from a marble to TON 618:

• Earth’s mass as a black hole: about 9 mm, the size of a marble.
• The Sun’s mass: about 3 km, small enough to fit inside a city.
• Sagittarius A*, the Milky Way’s central black hole (4.3 million Suns): about 12.7 million km, roughly 18 times the Sun’s radius.
• TON 618 (40.7 billion Suns): about 120 billion km, nearly 9,500 times wider than Sagittarius A*.

Try any mass in the Schwarzschild Radius Calculator, which uses the same function as this tour, so the two always agree.

Sagittarius A* size compared to the Sun

Sagittarius A* is the supermassive black hole at the center of our galaxy, about 26,000 light-years away. Its event horizon is about 18 times the Sun’s radius (0.085 AU). That sounds enormous, but placed where the Sun is, it would reach only about a fifth of the way to Mercury’s orbit. In the tour, the Sun stays drawn as a small outline inside it.

The largest stars: UY Scuti and Stephenson 2-18

The largest known stars are cool, swollen red supergiants. Their sizes are hard to pin down, because they depend on the stars’ distances, which are uncertain, and on where you put the edge of a fuzzy, extended atmosphere.

• UY Scuti: about 909 times the Sun’s radius (4.2 AU), based on the Gaia DR3 distance. The long-quoted 1,708 R☉ assumed the star was much farther away. At 909 R☉ it would engulf Mercury, Venus, Earth and Mars.
• Stephenson 2-18: about 1,840 times the Sun’s radius (8.6 AU), from a 2026 study, with a range of roughly 1,480 to 2,150 R☉. The widely repeated 2,150 R☉ would put its surface just past Saturn; the newer value puts it between Jupiter and Saturn. Its distance alone may be uncertain by up to 50%.

Even the largest star is tiny next to the biggest black holes. TON 618’s event horizon is about 94 times wider than Stephenson 2-18.

Why black holes and stars are drawn differently

A black hole has no surface, so the tour draws its physical size: the event horizon.

• The dark disc is the event horizon, at its true size: the boundary beyond which nothing, not even light, can escape. It is the size this page compares and the one the numbers above refer to.
• The thin bright ring at its edge stands in for the photon ring, light that skims the black hole on its way to us. It is brighter on one side, because glowing gas moving towards us looks brighter than gas moving away. It is not an accretion disc, which in real black holes can extend much farther out.
• The stars behind it are bent around the disc by its gravity, which is why they appear stretched into arcs near the ring. This gravitational lensing is drawn with the simple point-mass formula, so treat it as illustrative.

A telescope would see something bigger. Gravity bends light so strongly that a black hole casts a dark shadow about 2.6 times wider than its event horizon, with the photon ring at the shadow’s edge. That shadow is what the Event Horizon Telescope images of M87* and Sagittarius A* show. It is an image, though, not an object: nothing physically extends that far out. Gas can orbit a non-spinning black hole stably as close as 3 event-horizon radii, only just outside where the shadow’s edge appears. Since this page compares physical sizes against real orbits, it draws the horizon.

The stars are drawn as simple glowing spheres, darker towards their edges. Red supergiants have no crisp surface: their outer layers are vast and thin, so they are drawn with soft, fuzzy edges, while the Sun’s edge is sharp. The planets’ orbits are drawn as seen from slightly above the orbital plane; the scale bar measures true distance along the horizontal. At TON 618’s step, only M87* is shown inside it, since the Solar System would be a speck.

Real black holes spin, and a spinning black hole’s horizon is smaller, down to half the Schwarzschild radius at maximum spin. The Black Hole ISCO Calculator shows how spin changes the horizon and the innermost stable orbit. For how long these giants would take to evaporate through Hawking radiation, see the Black Hole Evaporation Time Calculator.

Sources

• TON 618: 4.07 × 10¹⁰ M☉ (Ge et al. 2019, C IV line); older 6.6 × 10¹⁰ M☉ from the Hβ line.
• M87*: 6.5 × 10⁹ M☉, Event Horizon Telescope Collaboration (2019).
• Sagittarius A*: 4.297 × 10⁶ M☉, GRAVITY Collaboration (2022).
• UY Scuti: 909 R☉, Healy et al., photometry with the Gaia DR3 distance; older 1,708 R☉ from Arroyo-Torres et al. (2013).
• Stephenson 2-18: 1,840 (+310/−360) R☉, Siebert et al. (2026); older 2,150 R☉.
• Orbits: NASA planetary fact sheets. Voyager 1’s distance is approximate for late 2026.